Overtemperature protected apparatus



y 2 D. J. M GREGOR 3,036,242

' OVERTEMPERATURE PROTECTED APPARATUS Filed June 1'7. 1958 I 2 Sheets-Sheet 1 50 WITNESSES Fig. 2 53 MENTOR M Dean J. Mac Gregor W BY 5 A ATTOR EY- May 22, 1962 D. J. M sREeoR OVERTEMPERATURE PROTECTED APPARATUS 2 Sheets-Sheet 2 Filed June 17. 1958 Temperature C Fig.3

United Pat The present invention relates to overtemperature protected apparatus, and more particularly to an overtemperature protective system for detecting overloads in current carrying windings of electrical apparatus.

Dynamoelectric machines have usually been protected against overheating by means of thermal overload relays having a thermally responsive element, usually a bimetallic element, heated either directly by the current of the protected machine, or by a heater carrying the current, so that the relay responds solely to the current. Relays of this type do not give entirely satisfactory or adequate protection, however, since the relay responds only to the current and the thermal characteristics of the relay do not, and obviously can not, match those of the machine, so that the thermal response of the relay differs from that of the machine. Thus, the relay is affected differently than the machine by changes in the ambient temperature, and the response of the relay may also be affected by heat from other adjacent devices, or by air currents and similar conditions which do not affect the machine.

At present, nearly all overload relays have the same defect. They are designed to give an action in response to a signal generated as a result of some phenomenon occurring outside the apparatus they are supposed to protect. In all motor installations, a major problem is to prevent internal damage to the windings due to excessive heat. Therefore, it is desired to provide an inherent overload device that will be activated directly by the 'heat of the motor winding.

A protective system capable of operation in this manner is disclosed and claimed inthe copending application of G. P. Gibson and J. G. Schwarckopf, Serial No. 710,111, filed January 20, 195 8, assigned to the Westinghouse Electric Corporation. In this system, the overternperature sensing element is a temperature sensitive resistance element having a low temperature coefiicient of resistance in the normal operating temperature range of the device to be protected, but a very high temperature coefficient when the temperature exceeds the safe operating temperature (see FIGURE Various control circuits are described in the above-mentioned application for actuating a relay in response to a change in temperature of the resistance elements. In the systems referred to above, the sensing units, which include one or more temperature sensitive resistance elements, often called thermistors, are embedded in the motor windings or placed in intimate contact with the device to be protected. The sensing unit is connected in series with a coil for actuating a protective relay and a source of power for energizing the coil. A main contactor is usually provided for connecting the device to be protected to its source of power. The main contactor is generally actuated by a coil which is energized from a separate source of power. The contacts of the protective relay are in series with the main contactor actuating coil and its source of power. When the contacts of the protective relay are closed, the power source supplies current to the main contactor actuating coil, thereby permitting closing of the main contactor to connect the device to be pro tected with its source of power. There is ordinarily a separate switch in series with the coil in order that the device to be protected maybe connected or disconnected manually without regard to the overtemperature protective relay contacts. The temperature sensitive resistance elements in the above-described system may have a positive temperature coefficient of resistance or a negative temperature 'coeflicient of resistance. Various control schemes are disclosed to coordinate with either type of resistance element. The circuitry varies in accordance with whether the resistance elements have a positive or a negative temperature coefi'icient of resistance. A normally open relay may be used with positive temperature coefficient resistance sensing elements because in this type of resistance element the resistance increases rapidly with a rise in temperature. Thus, at normal operating temperatures, the relay coil is energized thereby actuating the protective contacts to closed position since the resistance is low and suflicient current will flow through the coil to energize the relay. As the temperature rises, the resistance in the sensing elements rises and, therefore, there will not be sufficient flow of current to energize the protective relay. It will drop to its normally open position disconnecting the device to be protected from the line.

Some of the primary requisites of a protective system such as this are that it must follow the temperature of the device to be protected closely so that the protective relay will trip and disconnect the device from the line when the temperature exceeds a predetermined value; it should be capable of resetting itself and reconnecting the power supply to the device to be protected when the temperature drops low enough so that the apparatus can be started again without having the protective relay trip on the attempt; it should, in the case of a motor, be able to protect against single phasing and it must also be fail-safe, that is it must be capable of disconnecting the protected device from the line when the protective system has an open circuit or a short circuit or if for any reason the protective system is not energized.

Where the relay of the above-mentioned system is normally open, a failure of the power supply to its actuating coil will cause it to remain in the open position. Likewise, an open circuit in the protective system will also cause the relay to remain its normally open position. In this type of system, however, without additional circuitry, a short circuit in the protective system will energize the relay to its closed position, since sufficient current will flow through the coil to energize the normally open relay. Therefore, without additional elements, this type of relay lacks the fail-safe characteristic of disconnecting the device to be protected from the line when there is a short circuit in the protective system. In one example of the above disclosure, a solution to this difiiculty is disclosed. A fuse is placed in series with the protective relay coil. In case of a short circuit, there will be an overcurrent on the fuse and it will blow thus deenergizing the coil and permitting the relay to drop to its open position thereby disconnecting the device to be protected from the line. However, practical circuits have a short circuit current which is not too much larger than normal operating current. Thus, the fuse rating becomes critical if proper protection is to be attained.

In order to protect against single phasing, when this system is employed in protecting polyphase dynamoelectric machine, the positive temperature coefficient of resistance type of sensing element must be connected in series with the other sensing elements in the other phases of the machine. Single phasing will cause the windings of the remaining phases to overheat. When one or more of the elements overheat, the resistance in that element will rise to a point where there will not be sufiicient current flowing through the protective relay coil to energize it thereby causing the contacts to drop to their normally open position.

The principal object of the present invention is to provide an improved system of the type described above for protection against overtemperature with improved fail-safe characteristics.

Another object of the invention is to provide a protective system against overtemperature in which a positive means is provided for sensing short circuits and open circuits in the protective system as well as failure of the power supply to the protective system.

A further object of this invention is to provide a protective system against overtemperature in electrical apparatus having current carrying windings which senses directly the heat in the windings and which has improved fail-safe characteristics.

A further and more specific object of the invention is to provide an overtemperature protective system utilizing a relay having a high reactance coil supplied by an alternating current source through rectifiers whereby a short in the leads of the overtemperature protective system will decrease the coil current below the dropout value thus providing a tail-safe circuit if accidental damage occurs to the leads. a

An additional object of the invention is to provide an improved control and power supply circuit for thermistor protected devices by which the thermistor circuit, including the control power transformer where one is used, is energized from the main line ahead of main contactor but are energize-d and deenergized through the usual starting and other control contacts so that there is no line circuit at the protected device and there are no losses in the primary of the control transformer when one is used, during the time when the protected device is turned 01f.

Other objects and advantages of the invention will be apparent from the following detailed description, taken in connection with the accompanying drawings, in which:-

FIGURE 1 is a partial perspective view, partly broken away, showing a thermally protected machine;

FIG. 2 is a schematic diagram showing the thermally responsive device connected to a three-phase motor;

FIG. 3 is a schematic diagram showing another modification of the invention incorporated in'a three-phase motor wherein the device has a different form of power supply than the FIG. 2 modification.

FIG. 4 is a schematic diagram showing still another modification of the invention; and a FIG. 5 is a typical temperature-resistance curve of the thermally responsive resistance material utilized in this invention.

FIG. 1 shows a thermally protected electric motor embodying the invention. The invention is shown embodied in a three phase alternating current induction motor for the purposes of illustration but it will be understood that the protective system of this invention is generally applicable to dynamoelectric machines of any type, and to other electrical devices such as transformers, for example, in which a current carrying winding is desired to be protected against overtemperature. It is also applicable to other devices which are electrically operated and in which it is desired to disconnect them from a power source at a predetermined temperature and, if desirable, reconnect them at a lower predetermined temperature.-

The motor shown in FIG. 1 includes a frame structure in which is supported a stator core 12 with windings 14 disposed in slots in the core in the usual manner with the end turns of the windings extending beyond the core. The motor also has a rotor 16 mounted on a shaft 18 supported for rotation in bearings 20 carried in suitable end brackets 22. Shown embedded in the end extension of winding leis a temperature sensing unit 24 having temperature sensitive resistance elements or thermistors 24a, 24b and 24c connected in series. Leads 25 extend from each'end of the sensing unit 24. The leads 25 may be brought out to a junction box (not shown), if desired, for connection with the remainder of the protective circuit. The sensing unit 24- comprises three thermally sensitive semiconductor devices which change resistance rapidly as their temperature changes over a small predetermined range. v

The particular resistance elements or thermistors shown have a positive temperature coeflicient of resistance in the temperature range in which it is desired to protect the motor. An example of a suitable resistance material is described in the copending application Serial No. 717,191, filed February 24, 1958, by Y. Ichikawa and assigned to the Westinghouse Electric Corporation, and now Patent No. 2,976,505. A curve illustrating the temperature resistivity characteristics of one example of a resistance material suitable for use as a sensing element in this invention is shown in FIG. 5. It can be seen that for a range of temperature below a predetermined value which would be the normal operating temperature of the device to be protected the resistance is substantially constant. At a given temperature the curve rises sharply indicating an extremely rapid rise in resistance with rise in temperature. Thus, the curve shows that the temperature sensitive resistors, or thermistors, have a relative low temperature coeflicient of resistance at normal temperatures compared with the much higher temperature coefiicient of resistance at elevated temperatures above a predetermined value. The values shown on the curve are for purposes of illustration. It will be understood that the temperature at which the resistance beings to rise rapidly can be preselected by selecting the desired composition of material. The range of temperatures indicated by A is the allowable operating range for the protected device. The range of temperature indicated by B is the overheating range. When the temperature reaches point C the protective system of this invention is designed to disconnect the device to be protected from the power supply as will be hereinafter described.

It will, of course, be understood that for some applications where all of the advantages hereof are not required negative temperature coefiicient of resistance elements may also be used, provided that corresponding changes be made in the remainder of the circuitry. For example, it would be necessary to connect negative temperature coefiicient of resistance elements in parallel if it is desired to sense single phasing in a motor. Further, a normally closed relay must be used. Thermally sensitive resistance elements can be built in various sizes but the ones that are preferred for use in this invention are of the order of .0625 inch long and .25 inch in diameter with a small size lead wire brought out at each end. Being so very small, these resistors can easily be located in the windings of a motor, so that they can very closely follow the temperature cycle of the copper with the heat being transmitted through the conductor insulation to the sensing unit 24 and the motor conductor insulation providing the necessary electrical insulation so that the resistance element will not have motor voltage applied to it.

FIG. 2 shows a manner in which one embodiment of the protective device is electrically connected for use in the stator windings of a three-phase induction motor. The temperature sensing unit 24 comprises three individual resistance elements 24a, 24b and 240 connected in series. Each of the resistor sensing elements is embedded in a phase winding of the motor as shown in FIGS.

1 and 2. This embodiment shows a three-phase motor having phase windings 30a, 30b and 30c, connected to a three-phase line 32 by means of a contactor 34 of any suitable type. The contactor 34 is actuated by a coil 36 which is connected across one phase of the line 32 and controlled by a start pushbutton 38 which energizes coil 36, which in turn closes the three-pole contactor 34-, the contactor being held in by a sealing contact 4%, which is also actuated by coil 36, connected in parallel with the starting switch 38. The motor is stopped by actuation of the stop pushbutton 42 which has contacts in series with the start pushbutton 38. The motor may also be stopped by the opening of a pair of contacts 44 of relay device 45 having two conditions, the operation between which will .5 be explained hereinafter. The contacts 44 are in series with the contacts of stop pushbutton 42 and start pushbutton 38 and coil 36. Either actuation of stop pushbutton 42 or opening of contacts 44 interrupts the circuit of coil 36 and allows the contactor 34 to open.

Contacts 44 of device 45 in one condition thereof are normally open and in the other condition, the contactor-s are actuated to the closed position by energization of the coil 46. Protective relay coil 46 of protective relay 45 is connected in a series circuit comprising transformer secondary winding 48, the three temperature sensitive resistance elements 24a, 24b and 240 and rectifier 49 which is preferably located within the motor housing. Any suitable type of rectifier 49 may be used such as, for example, an appropriately rated diode. A capacitor 51 is connected across the relay coil 46. The relay coil 46 and capacitor 51 are connected at one side of transformer secondary 48 by a lead 47 and at their other side to one side of the temperature sensing unit 24 by a lead 52. The rectifier 49 is connected at one side to the other side of the transformer secondary 48 by a lead 53 and at its other side to the other side of the temperature sensing unit 24 by a lead 55. The capacitor 51 serves to prevent the relay from dropping open during the half-cycle in which no current is flowing. It also serves to prevent chatter when the coil current is maintained at a value slightly below the amount necessary for pickup. The primary winding 54 of transformer t! is shown connected across one phase of the three phase line 32. It will, of course, be understood that any suitable alternating current source of power supply may be employed.

The operation of the protective system should now be apparent from the above detailed description. The motor is started by momentarily closing the button 38 which energizes the coil 36 and closes the contactor 34 and the sealing contact 40. At normal operating temperature, the resistance of the elements 24a, 24b and Me is very low and sufficient current flows to energize coil 46 which in turn actuates the contacts 44 to closed position permitting con tactor 34 to close and start the motor when the start button is pressed. The elements 24a, 24b and 240 are embedded in the windings, one in each phase. Upon overheating the combined resistance of the sensing unit 24 increases rapidly causing the relay 45 to ecome deenergized which, in turn, deenergizes the coil 36 opening the line contactor 34, thereby removing power from the motor.

If, for some reason the sensing unit 24, the relay coil 46, or the power supply is not in the protection circuit, the motor will not start. When the temperature of the wind ings falls to a predetermined value, the resistance of the elements 24a, 24b and 240 will drop to a low value again, thereby energizing coil 46 and actuating contacts 44 or" protective relay 45 to closed position. The starting circuit for the motor is then again in position to start and momentary actuation of pushbutton 38 will restart the motor. If one phase of the line 32 should be open and the motor begins to single phase, the remaining windings will heat up thereby heating the associated temperature sensitive elements of unit 24, increasing its resistance to a point Where there will not be sufficient current to energize the coil 46. In this case, the contacts 44 will again drop open. An open circuit in the lead lines 47, 52, 53, or 55 will again deenergize the coil 46 thereby disconnecting the motor from the line 32.

A failure in transformer 56 or a loss of power supply to transformer St) will again deenergize coil 46 and again the motor will be disconnected from the line. Should a short occur between leads 53 and 47, the secondary winding 48 will be shorted out and no power will be supplied to the coil 46 and again, in this instance, the contacts 44 will open disconnecting the motor from the line 32. Should a short occur between leads 52 and 53 running between the controller and the motor, the rectifier 49 as well as-the temperature sensing unit 24 will be inoperative.

Relay coil 46, tits, is supplied with alternating current and the high impedance of the coil 46 limits the current to a value below that necessary to energize relay 45. Again, contacts 44 will open disconnecting the motor from the line 32. Since rectifier 49 is placed within the motor housing lead 52 can be adequately protected from shorting to lead 55. By mounting the rectifier 49 adjacent the motor, the lead 55 is kept very short and is protected. This has an advantage over placing the rectifier in the controller near the transformer 50, since in that case the lead 55 would have to extend with conductor 52 the whole distance from the controller to the motor. Thus, the danger of a short circuit between the conductors 52 and 55 would be greatly increased and such a short circuit would not cause opening of the protective relay 45 since there would then be direct current flowing in the coil 46 of the relay. This difiiculty is avoided by putting the rectifier at the motor where the possibility of a short circui-t between the very short conductor 55 and conductor 52 is minimized.

Thus, it can be seen that an overternperature protective system has been provided which will protect against overtemperature in an electrical device as well as single phasing of a polyphase motor, and which is fail-safe against shorts or opens in the protective circuit and loss of power supply to the protective circuit. It is a system which will reset itself upon return to normal operating temperature of the device which is to be protected.

Another modification of the invention is illustrated in FIG. 3. The FIG. 3 embodiment is illustrated for use in a three phase star-connected motor similar to that of FIG. 2. Temperature sensitive resistance elements 24a, 24b and 24c which constitute a temperature sensing unit 24 are each embedded in one phase winding Sila, 30b and 300 of the motor stator. The phase windings 30a, 3% and 300 are each connected to one phase of a three phase power supply 32 through a three-pole contactor 34 which is actuated by a coil 36. The contactor 34 is energized by a coil 36 which is connected across one phase of the line 32 through a start pushbutton 38 and a stop pushbutton 42. A sealing contact 44) actuated by the coil 36 is connected across the contacts of start pushbutton 33. A protective relay 45 has normally open contacts 44 connected in series with the contacts of start pushbutton 3d and stop pushbutton 42. Contacts 44 of protective relay 45 are actuated by a coil 46. The protective system includes a transformer 50 having a secondary winding 48 and a primary winding 54 and the primary winding of transformer 5% is shown as connected across one phase of the three-phase line 32. It will be understood, of course, that any suitable source of power may be utilized to supply the primary winding 54 of transformer 54 The secondary 48 of transformer 5b is center tapped as at 57 by center tap lead 59. Coil 46 is connected at one side to center tap 57 by lead 59. At its other side it is connected to one side of sensing element 24 comprising sensing elements 24a, 24b and 240 connected in series by a center tap lead 5%. To each side of the transformer secondary 48 is connected one of the like terminals of rectifiers 56 and 58, through leads 61 and 63 respectively. Rectifiers 56 and 58 are connected at their other lilte terminals to a common point 64). A lead 65 connects point 69 with the other side of sensing unit 24.

Thus, it can be seen that the protectivesystem shown in FIG. 3 consists of a single-phase full-wave rectifier including a center tapped transformer secondary winding 48, a pair of rectifiers 56 and 53 which may be of any suitable or desirable type as, for example, diodes of the proper rating, in which the center tap includes a coil 46 and the temperature sensing elements 24a, 24 and 240 in series connected at a common point 60 with one side of each of the rectifiers 56 and 58.

The FIG. 3 modification, since it employs a full-wave rectifier instead of a half-wave rectifier, eliminates the need for capacitor 51 disclosed in the FIG. 2 modification.

Its operation is quite similar to that of the previously described modification. When the start pushbutton 38 is pressed, a closing coil 36 is energized closing contactor 34 thus energizing the motor. The sealing contact 40 is also closed by energization of coil 36. When the temperature of the motor is low, the resistance of the sensing unit 24 is also low and relay 45 is energized closing contacts 44. This allows normal operation of the conventional motor starting apparatus. If the motor temperature rises above a predetermined safe value, the resistance of sensing unit 24 increases sharply, deenergizing relay 45 and opening contacts 44, thereby deenergizing coil 36 and disconnecting the motor from the line 32.

Fail-safe protection is provided since relay coil 46 must be energized before the motor can start and opens in the protection circuit will deenergize the relay 45. Furthermore, relay 45 is designed to operate not on peak current but on average direct current and if either transformer lead 61 or 63 is open the current will drop below the value necessary to actuate the relay 45. The effective current will be approximately half of the effective current with no open leads, since the circuit is now a half wave rectifier. Also the relay presents a greater impedance to the half wave current than to full wave because of the filtering action of the coil inductance. The combination of the decrease in the average value of current and increase in coil impedance is enough to deenergize the relay. When the center tap 59 is open, of course, no current flows through the relay coil 46. A short between the transformer leads 61 and 63, of course, shorts out the transformer from the circuit and no current is supplied to the coil 46. When the center tap 59 and either of the transformer leads 61 or 63 are shorted, a portion of transformer winding 48 is shorted and again no current is supplied to the coil 46. A short between leads 61 or 63 and 59a allows alternating current to flow in the coil 46. Relay 46 then drops open since the coil impedance to AC. is considerably greater than to DC. and therefore, the current decreases below the dropout value. Note that the leads between the controller and the motor are especially vulnerable to shorts; therefore, the rectifi'ers are preferably placed in the motor housing.

Therefore, the FIG. 3 modification, like the FIG. 2 embodiment of this invention, disconnects the motor from the line due to excessive temperature of a stator Winding, single phasing, opens in the power supply to the protective system, opens in the protective system and shorts in the protective system. Thus, it is completely fail-safe and is also capable of resetting when the temperature of the motor returns to a normal operating value.

Still another embodiment of the invention is illustrated in FIG. 4. In the embodiments described hereinabove it can be seen that transformer 56, relay 45 and sensing unit 24 are energized all the time even though the motor or other protected device may not be energized. It may be desirable under certain circumstances to deenergize the transformer 59 when the device to be protected is not energized. This is accomplished by employing the circuit shown in FIG. 4.

The protective circuit of the FIG. 4 modification is identical to that of either the FIG. 2 or FIG. 3 embodiment. In order to simplify the drawings and avoid unnecessary repetition, only as much of the protective circuit is shown in FIG. 4 as will be essential to understand the difference between this embodiment and the previously described embodiments. Although the portion of the protective circuit shown is a portion of the FIG. 3 modification it will be obvious that the protective circuit of \FIG. 2 can be employed in this modification equally as well. Again the invention is illustrated employed in a three phase motor for purposes of illustration. The phase windings are each connected to one phase of a three phase power supply through a three-pole contactor 34 which is actuated by a coil 36 which is connected across one phase of the line 32 through a start pushbutton 38 and a stop pushbutton 42. A sealing contact 40 actuated by coil 36 is connected across the contacts of start pushbutton 38. .Protective relay 45 has normally open contacts 44 connected in series with the contacts of the start pushbutton 38 and stop pushbutton 42. Contacts 44 of protective relay 45 are actuated to closed position by energization of coil 46. The protective system includes transformer 50 having a primary winding 54 and a secondary winding 48. The primary winding 54 is connected in parallel with coil 36 and relay contacts 44 and in series with the start button contacts 38 so as to be energized or switched on when the start contacts are closed. Herein lies the difference between the instant embodiment and the previously described embodiments. Initially, the transformer 50 is not energized. Therefore, the protective circuit is without power and contacts 44- remain open. When the start button is depressed current flows through the start button contacts, the stop button contacts and transformer primary 54. Since contacts 44 are open, current will not immediately flow through coil 36. However, upon energization of transformer 50 contacts 44 are caused to close thereby completing the circuit through coil 36 and thereby energizing the motor simultaneously with energization of the protective circuit. The remainder of the circuit and the operation thereof is identical to the above described embodiments and includes all of the fail-safe features which apply thereto. This circuit has the advantage that the protective circuit is deenergized so that there is no power loss therein and there are no live conductors at the motor when the motor is turned oil? by its regular control.

Although the system has been described employing positive temperature coefiicient resistance elements, it will be apparent that negative temperature coefiicient resistance elements could be substituted for the sensing elements 24 by connecting them in parallel instead of in series for single phasing protection and by substituting a normally closed relay for relay 45. Of course, by doing this some of the fail-safe characteristics would be lost.

It should now be apparent that inherent thermal protection has been provided by means of a relatively simple and inexpensive thermally responsive protective system which is applicable to any type of dynamoelectric machine including polyphase or single phase motors, to alternating current or direct current motors, to generators and to other electrical devices. It should be understood although the invention is described for use in a motor that this temperature sensing system can be applied to electrical devices other than dynamoelectric machines. It may be applied to transformers, for example. This system is applicable in any electrical device wherein it is desired to sense a temperature to actuate a relay in response to a change in temperature.

Certain preferred embodiments of the invention have been shown and described for the purpose of illustration but since various other embodiments and modifications are possible Within the scope of the invention, it is to be understood that the invention is not limited to the specific details or constructions shown, but in its broadest aspect, it includes all equivalents, embodiments and modifications.

I claim as my invention:

'1. In a dynamoelectric machine including polyphase current carrying windings, a main contactor connecting said windings to a source of current, a coil for actuating said main contactor, a main contactor control circuit comprising a power source and a starting switch connected in series with said coil; an overtemperature protective system comprising a plurality of resistance elements connected in series and having at a predetermined temperature a high positive temperature coefficie-nt of resistance, an element embedded in each phase of said windings, a protective relay having normally open contacts in series with said main contactor coil, a pro-tective relay coil for actuating said contacts to closed position when said coil is energized, said protective relay coil having high reactance relative to its direct current resistance such that it is effectively energized when direct current is applied and not energized when alternating current is applied, said coil, said resistance elements, rectifier means and an alternating current source connected in series, and said rectifier means connected in close proximity to said resistance elements.

2. In a dynamoelectric machine including polyphase current carrying windings within an enclosure, a main contactor connecting said windings to a source of current, a coil for actuating said main contactor, a main contactor control circuit comprising a power source and a starting switch connected in series with said coil and plurality of resistance elements of semiconductor material connected in series and having a relatively fiat, lovr resistance curve over a substantial temperature range up to a predetermined overload temperature and a sharp rise in resistance above said predetermined temperature, one of said elements embedded in each phase of said windings in close thermal relation therewith; an overtemperature protective system comprising a normally open direct current operated protective relay having contacts in series with said main contactor coil, a coil for actuating said relay to closed position when energized, said protective relay coil having high reactance relative to its direct current resistance such that it is not eifectively energized when alternating current is applied, a single-phase full-wave rectifier, including a center tapped transformer secondary winding, a pair of diodes located within said enclosure in close proximity to said resistance elements, one of said diodes connected at one side thereof to one side of said secondary winding, the other of said diodes connected at one side thereof to the other side of said secondary winding, said center tap connected in series with said protective relay actuating coil and said resistance elements to a common point with the other side of said pair of diodes.

3. An overtemperature protective system for protecting a winding comprising a resistance element having a relatively low temperature coefficient of resistance at normal temperatures and a much higher positive temperature coefiicient of resistance when a predetermined abnormal temperature is approached disposed in heat transfer relationship with said winding, a protective relay including a coil having high reactance relative to its direct current resistance such that it is efiectively energized when direct current is applied and not effectively energized when alternating current is applied, said relay having contacts biased open and closed when energized, an alternating current means for supplying said protective system, rectifier means connected in series with said alternating current means and said coil and said resistance element, said rectifier means being physically disposed adjacent said resistance element and at a distance from said core, said coil being energized when the temperature of said resistance element is within a preselected range at which the resistance of said element is of a value low enough to provide sufficient direct current through said coil to actuate said relay and being efiectively deenergized if there is either a break or a short circuit in the circuit between said coil and said rectifier means.

4. An overtemperature protective system for protecting a polyphase winding comprising a plurality of resistance elements connected in series, each having a positive temperature coefiicient of resistance when a predetermined abnormal temperature is approached and disposed in heat transfer relationship with said winding, a protective relay including a coil having high reactance relative to its direct current resistance such that it is effectively energized when direct current is applied and not effectively energized when alternating current is applied, said relay having contacts biased open and closed when energized, an alternating current connection for supplying said protective system, rectifier means connected in series with said alternating current connection and said coil and said resistance elements, said rectifier means being physically disposed adjacent said resistance elements, whereby said coil is energized only when the rectifier means is in the circuit and the temperatures of all of said resistance elements are within a preselected range at which the resistance of said elements is of a value low enough to provide sufiicient direct current through said coil to actuate said relay.

5. An overtemperature protective system for protecting a winding enclosed within a housing, a resistance element of semiconductor material having a relatively low resistance curve over a substantial temperature range below a predetermined overload temperature and a sharp rise in resistance above said predetermined temperature, said element embedded in said winding in good heat transfer relationship therewith, a protective relay including a coil having high reactance relative to its direct current resistance such that it is effectively energized when direct current is applied and not eifectively energized when alternating current is applied, said relay having contacts biased open and being closed when energized, an alternating current circuit for supplying said protective system, rectifier means connected in circuit with said alternating current circuit and said coil and said resistance element, said rectifier means being enclosed at said housing.

6. In a dynamoelectric machine including polyphase current carrying windings within an enclosure, a main contactor connecting said windings to a source of current, a coil for actuating said main contactor; an overtemperature protective system comprising a plurality of resistance elements connected in series and having at certain temperatures a high positive temperature coefficient of re sistance, one of said elements embedded in each phase of said windings in close thermal relation therewith, a protective relay having normally open contacts connected to control said main contactor coil, a protective relay coil for actuating said contacts to closed position when said protective relay coil is energized, said protective relay coil having high reactance relative to its direct current resistance such that it is efiectively energized when direct current is applied and not effectively energized when a lternating current is applied, rectifier means connected in circuit with said protective relay coil and said resistance elements and an alternating current supply circuit, said rectifier means disposed in close proximity to said resistance elements within said enclosure.

7. In a dynamoelectric machine including polyphase current carrying windings within an enclosure; an overtemperature protective system comprising a plurality of resistance elements connected in series and having a positive temperature coeificient of resistance, one of said elements being mounted adjacent each phase of said windings in thermal relation therewith, said elements being of semiconductor material and having a relatively low resistance curve over a substantial temperature range below a predetermined overload temperature and a sharp rise in resistance above said predetermined temperature, a protective relay having normally open contacts and positioned at a distance from said windings and resistance elements, said protective relay having a coil for actuating said contacts to closed position when said protective relay coil is energized, said protective relay coil having high reactance relative to its direct current resistance such that it is eliectively energized when direct current is applied and not eitectively energized when alternating current is applied, rectifier means, circuit means for connecting said protective relay coil and said resistance elements and rectifier means to an external alternating-current circuit, said rectifier means being connected in the circuit at a greater distance from the protective relay coil than from said resistance elements, and said protective relay coil being sutficienlty deenergized to open the relay contacts upon either a break or a short circuit in the circuit between the relay coil and rectifier.

8. In a dynamoelectric machine including a stator core having current carrying windings subject to overheating under certain abnormal conditions, said machine having a frame and end brackets forming'an enclosure for the machine, a temperature responsive resistor having a positive temperature coeflicient of resistance disposed in direct thermal contact with said windings, rectifier means disposed within said enclosure and connected to said resistor Within the enclosure, and circuit means for connecting said resistor and rectifier means to an external protective circuit including an alternating current source, said external protective circuit including high reactance circuit means and being effectively energized only by fiow of direct current therethrough in either one direction or the other direction.

9. In a dynamoelectric machine including a stator core having current carrying windings subject to overheating under certain abnormal conditions, said machine having a frame and end bracket forming an enclosure for the machine, a temperature responsive resistor having a positive temperature coefficient of resistance disposed in thermal relation with said windings, said resistor being of semiconductor material and having a relatively low resistance curve over a substantial temperature range below a predetermined overload temperature and a sharp rise in resistance above said predetermined temperature, rectifier means enclosed at said enclosure "and connected to said resistor within the enclosure, and circuit means for connecting said resistor and rectifier means to an external protective circuit including an alternating current source, said external protective circuit including high reactance circuit means and being effectively energized by direct current flowing therethrough in either one direction or the other direction.

10. In a dynamoelectric machine including a stator core having current carrying windings subject to overheating under certain abnormal conditions, said machine having a frame and end brackets forming an enclosure for the machine, a temperature responsive resistor having a positive temperature coefficient of resistance disposed in thermal relation with said windings, said resistor being of semiconductor material and having a relatively low temperature coefficient of resistance over a substantial temperature range below a predetermined overload temperature and a relative high temperature coefiicient of resistance above said predetermined temperature, rectifier means enclosed at said enclosure and connected to said resistor within the enclosure, a protective relay including a coil connected to be energized by current through said resistor, said coil having high reactance relative to its direct current resistance such that it is effectively energized when direct current is applied and not effectively energized when alternating current is applied, said relay having contacts biased open and being closed when energized, and circuit means for connecting said resistor and said rectifier means and said relay coil to an external alternating current circuit.

11. In a dynamoelectric machine including polyphase current carrying windings, a main contactor connecting said windings to a source of current, a coil for actuating said main contactor and a power source for said main contactor circuit; an over-temperature protective system comprising a plurality of resistance elements of semiconductor material, one of said elements embedded in each phase of said windings in close thermal relation therewith, said elements having a relatively low resistance curve over a substantial temperature range below a predetermined overload temperature and a sharp rise in the resistance curve above said predetermined temperature, a protective relay having normally open contacts in series with said main contactor coil, a protective relay coil for actuating said contacts to closed position when said protective relay coil is energized, said protective relay coil having high reactance relative to its direct current resistance such that it is effectively energized when direct current is applied and not elfectively energized when alternating current is 12 applied, said protective relay coil, said resistance elements, rectifier means and an alternating current source being connected in series, said rectifier means disposed in close proximity to said resistance elements.

12. An electrical apparatus including a current carrying winding, a main contactor for connecting said winding to a line, a coil for actuating said main contactor, a starting switch connected in series with said coil to efiect energization thereof, one or more positive temperature coefiicient thermistors of semiconductor material subjected to the heating of said current carrying Winding, an electrical device having a high reactance relative to its direct current resistance and rectifier means connected in circuit with said one or more thermistors and having its energization changed to effect a change from one condition to another upon heating and a predetermined increase of resistance of said one or more thermistors, a control power supply circuit for said device and said one or more thermistors connected to the line ahead of said main contactor, one or more contacts in said control power supply circuit with one of said contacts being part of said starting switch, said one or more contacts energizing said control power supply circuit upon actuation of the starting switch to energize said coil to close the main contactor, said one or more contacts being operable to deenergize said control power supply circuit and the circuit of said device and of said one or more thermistors when the main contactor is opened, and a break in the circuit between said device and said one or more thermistors causing a change in condition of said device like said predetermined increase in resistance of said one or more thermistors.

13. An electrical apparatus including a current carrying winding, a main contactor for connecting said winding to an alternating-current line, a coil for actuating the main contactor, a transformer having a single-phase primary connected to be energized from an alternatingcurrent line independently of the actuation of the main contactor and having a single-phase secondary with a center tap, a thermistor mounted adjacent said winding to be heated by the alternating current flow in said winding, a pair of rectifiers having like ends connected together and to one end of said thermistor, a connection from the other end of each rectifier to one of the ends of said single-phase secondary, a device having high reactance relative to its direct current resistance which changes from one condition to another upon a change in energization connected between said thermistor and the center tap of said single-phase secondary, contact means for controlling the energization of said coil for actuating the main contactor including a start switch having contacts operable to energize said single-phase primary and to effect energization of said coil of the main contactor, and said contact means effecting deenergization of said singlephase primary when said coil of the main contactor is deenergized and said start switch contacts are open.

14. An electrical apparatus including a winding for carrying load current, an electrically controlled main switching device for connecting said winding to an altermating-current line, a transformer having a single-phase primary connected to be energized from an alternatingcurrent line at least when said main switching device is closed, said transformer having a single-phase secondary with a center tap, a thermistor mounted adjacent said winding and heated by heat generated in said winding by the alternating-current flow therein, said thermistor having a relatively small temperature coelficient of resistance at normal temperatures and a much larger coefficient when a predetermined abnormal temperature is approached, a pair of rectifiers having like ends connected together and to one end of said thermistor, a connection from the other end of each rectifier toone of the ends of said single-phase secondary, a circuit device which changes from one condition to another having a high reactance relative to its direct current resistance connected at one end to the center tap of said single-phase secondary and at the other end to the other end of said thermistor with said circuit device carrying current when the circuit is complete and unshorted which flows in only one direction and is the same current at all times as flows through the thermistor, a non-polarized armature actuated upon a predetermined change in the magnitude of the current flow in said one direction through the circuit device as said thermistor is heated and its resistance changes, and contact means for controlling the electrical operation of said main switching device.

15. In a dynamoelectric machine including a stator core having current carrying windings subject to overheating under certain abnormal conditions, said machine having a frame and end brackets forming an enclosure for the machine, a temperature responsive resistor having a posirtive temperature coefficient of resistance disposed in thermal relation with said windings, said resistor being of semiconductor material and having a relatively low resistance curve over a substantial temperature range below a predetermined overload temperature and a sharp rise in the resistance curve above said predetermined temperature, a full wave rectifier comprising a pair of diodes having like ends connected together and to one end of said resistor of semiconductor material and said diodes having their other ends connected to an alternating current supply circuit, a protective relay including a coil connected to the other end of said resistor of semiconductor material and to the alternating current supply circuit and energized solely by the entire current through said resistor, said coil having high reactance relative to its direct current resistance and thereby being effectively energized only when direct current is applied through said diodes from said alternating current supply circuit in either one direction or the other direction, said relay having contacts biased open and being closed when efiectively energized, and circuit means for connecting said relay contacts to an external circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,106,100 Jones Jan. 18, 1938 2,150,071 Landis Mar. 17, 1939 2,283,706 Stimson May 19, 1942 2,446,474 Hanold Aug. 3, 1948 2,681,429 Long June 15, 1954 2,721,993 Medlar Oct. 25, 1955 2,758,255 Lytle Aug. 7, 1956 FOREIGN PATENTS 522,731 Belgium Sept. 30, 1953 735,755 Great Britain Aug. 31, 1955 

